The present invention relates to a method for forming an oxide, and more particularly to a method for forming a field oxide.
The active area of the metal oxide semiconductor (MOS) device is generally isolated with the field oxide formed by the local oxidation of silicon (LOCOS) in the manufacturing process of integrated circuit (IC) above 0.25 μm. FIGS. 1(a)–(d) are schematic diagrams showing the conventional process for forming the field oxide of the MOS device. As shown in FIG. 1(a), the first step is providing a silicon substrate 11 and forming a pad oxide 12 and a nitride film 13 such as Si3N4 on the silicon substrate 11. A photolithographic process is performed thereupon to form a patterned photoresist 14 on the nitride film 13, wherein the patterned photoresist 14 is formed via the pattern transfer of a mask to define the active area of the MOS device. As shown in FIG. 1(b), the nitride film 13 and the pad oxide 12 which are not covered with the photoresist 14 are subsequently etched and removed. As shown in FIG. 1(c), the photoresist 14 is removed and the resulted semiconductor structure is then placed into an oxidation furnace (not shown) to form a field oxide 16 with thermal oxidation. Finally, as shown in FIG. 1(d), the finished active area of the MOS device is isolated with the field oxide 16 after removing the pad oxide 12 and the nitride film 13. For isolating the neighbor active areas of the MOS device effectively, the field oxide 16 with a certain thickness is required.
With consideration of the oxidation reaction rate, the wet oxidation process is extensively used for forming the field oxide in industry nowadays. The chemical equation of the wet oxidation process is H2+O2+Si→SiO2+H2O, and the reaction condition and process are shown in FIG. 2. As shown in FIG. 2, the temperature of the furnace is raised from 700–800° C. to 800–1000° C. before the wet oxidation process. Subsequently, the reaction gas like hydrogen and oxygen with a flow ratio of 5500 sccm: 3300 sccm to 2000 sccm: 2000 sccm is introduced into the oxidation furnace to oxidize the silicon substrate 11 after the temperature of the oxidation furnace comes to a steady state. Finally, the temperature of the oxidation furnace is lowered down to finish the wet oxidation process after the certain thickness of the field oxide 16 is obtained and the reaction becomes stable.
While forming the field oxide 16, the active area of the MOS device is covered with the pad oxide 12 and the nitride film 13, therefore the reaction gas cannot permeate the nitride film 13 and react with the silicon substrate 11. The gas can only react with the silicon substrate 11 not covered with the nitride film 13 in an isotropic way. Due to the speedy reaction rate of the wet oxidation process, the field oxide forming rate at 45° and 90° angle is faster than that at 0° to 30° angle to the silicon substrate 11. Hence, it is easy to form the active area with a tip 111 as shown in FIG. 1(d) and FIG. 3 after removing the pad oxide 12 and the nitride film 13. The field concentration and tip discharge are attributed to the formation of the active area with a tip 111 and thus make a breakdown of the gate oxide, and further affect the quality and electric property of the MOS device.
Therefore, it is desirable to develop a method for forming an even field oxide without tip and dealing with the conventional defects.